Accurate knowledge of material thicknesses deposited on any solid state device is often of critical importance for precise control of the device's operation. This situation is true for all thin-film technologies but it is perhaps most true for a class of coatings known as optical interference coatings. These optical coatings are combinations of different transparent films deposited on a substrate to provide widely varying spectrally-dependent transmission and reflection properties. Optical coatings of many thin-film layers can be used to make narrow band reflectors, transmission filters for discrete wavelength selection, polarizers, and anti-reflection coatings. To obtain the desired interference effects, each coating must be precise in thickness to small fractions of a light wavelength, which in the visible spectrum is on the order of 0.4 to 0.7 microns.
Because of the need to know film thicknesses to a high accuracy, technology has been developed to monitor the thickness of deposited films during the actual deposition process. The technique most widely accepted and used for monitoring thickness during the deposition of optical films is called optical monitoring. The value of this technique lies principally in the fact that the light itself is being used to measure the film thickness. Since the optical properties are of the foremost concern, this technique comes closest to a direct measurement of the desired optical effects.
The underlying principle behind both optical monitoring and optical design is the phenomena of light interference. Light reflecting from each thin-film surface is coherent and interferes differently with light reflected from other surfaces, depending on the precise film thickness, to produce different optical effects. Different combinations of thin films of differing optical thicknesses and refractive indexes can be used to achieve a great variety of optical effects. A multilayer coating is successful if the optical element possesses the desired reflectance or transmittance values over the full useful spectral range. Conversely, the aim in optical monitoring during deposition is to produce the correct film thicknesses by understanding how the reflectance, at one distinct wavelength, is effected by film thickness.
A standard optical coating system consists of a large vacuum chamber (commonly called a box coater because of its shape) within which there are three essential elements: the material deposition source(s), a planetary arrangement holding the parts to be coated, and an optical thickness monitoring system centrally located in the vacuum system. The planetary system has a central axis around which additional planets having sub-axis can rotate. The parts to be coated may number anywhere from a few to many hundreds depending on system and part sizes and are mounted in holders in the planet positions. The parts rotate both about the planet axis and the central axis to maintain deposition quality on all parts within the system to average out any spatially dependent deposition differences. A stationary monitor chip or sample optical element, on which all optical coating measurements are made, is located at the central axis. Light from either a laser or a focused monochromatic light source is directed onto this monitor chip and then reflected back to a detector. As the film thickness on the monitor chip changes, the reflected signal varies.
In the optical industry it is not unheard of to need as many as 50 to 100 layers to produce a critical optical effect. An error in thickness of any of these layers can have dire consequences for the finished elements optical performance. Unfortunately, as the succeeding layers are deposited on the monitor chip the information learned about the thickness of the last layer diminishes readily because of the complex interference effects between all succeeding layers. In addition, the thickness sensitivity of the monitoring process to the last layer is significantly impaired. For example, in fabricating a narrow band reflector with ten high index/low index 1/4-wave thin-film pairs, the reflectivity might change from 4% to 37% after deposition of the first film pair on glass. Final reflectivity of 99.99% is realized after the tenth pair is deposited, but an undetectable reflectivity increase of only 0.01% occurs when the tenth and last thin-film pair is deposited. To avoid this problem it is a standard practice in the optical coating industry to periodically replace the coated chip with a clean chip during the deposition process.